A catalogue of errors over planning for Iraq after the invasion, and an inability to influence key figures in the US administration, led to anarchy in Iraq from which the country has not recovered, the British defence secretary during the invasion admits today.

In an exclusive interview with the Guardian, Geoff Hoon reveals that Britain disagreed with the US administration over two key decisions in May 2003, two months after the invasion - to disband Iraq's army and "de-Ba'athify" its civil service. Mr Hoon also said he and other senior ministers completely underestimated the role and influence of the vice-president, Dick Cheney.

Article continues "Sometimes ... Tony had made his point with the president, and I'd made my point with Don [Rumsfeld] and Jack [Straw] had made his point with Colin [Powell] and the decision actually came out of a completely different place. And you think: what did we miss? I think we missed Cheney."

Giving the most frank assessment of the postwar planning, Mr Hoon, admits that "we didn't plan for the right sort of aftermath".

"Maybe we were too optimistic about the idea of the streets being lined with cheering people. Although I have reconciled it in my own mind, we perhaps didn't do enough to see it through the Sunni perspective. Perhaps we should have done more to understand their position."

He said history would have to decide whether the coalition should have anticipated the Sunni-Shia violence. "Given what we know now, I suppose the answer is that we should, but we did not know that at the time."

Of the summary dismissal of Iraq's 350,000-strong army and police forces, Mr Hoon said the Americans were uncompomising: "We certainly argued against [the US]. I recall having discussions with Donald Rumsfeld, but I recognised that it was one of those judgment calls. I would have called it the other way. His argument was that the Iraqi army was so heavily politicised that we couldn't be sure that we would not retain within it large elements of Saddam's people."